Difficulties in domain wall nucleation have been shown to affect the domain state of the fine magnetic particles in rocks. Particles, large enough to contain a domain wall, remain saturated in remanence and only mucleate a wall when a backfield is applied. In this paper, nucleation is described during hysteresis, stress cycles and cycles through the low temperature anisotropy transition in magnetite. It is suggested that much of the grain size dependence of magnetic behaviour in the pseudosingle domain grain size range can be attributed to the increasing difficulty with which walls are nucleated, as particle size decreases.